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Amino Acids
Amino Acids are the building units of proteins. Proteins are polymers of amino acids linked together by what is called “ Peptide bond” (see latter).
There are about 300 amino acids occur in nature. Only 20 of them occur in proteins.
Structure of amino acids:
Each amino acid has 4 different groups attached to α- carbon ( which is C-atom next to COOH). These 4 groups are : α-amino group, COOH group, Hydrogen
atom and side Chain (R)
R
Classification of amino acids
1- Chemical classification: According to number of COOH and NH2 groups i.e. according to net charge on amino acid and
A- Monobasic, monocarboxylic amino acids i.e. neutral or uncharged:
R
According to chemical structure of R group, neutral amino acids are subclassified into:
I- Aliphatic, neutral amino acids: (6 amino acids)
1- Glycine (Gly): R= H
2- Alanine (Ala) R= CH3
3, 4, 5-Branched chain amino acids: valine, leucine and isoleucine R is branched Valine (Val): R= isopropyl group
Leucine (Leu): R= isobutyl gp
Isoleucine (Ile) R = is isobutyl
R is isobutyl in both leucine and isoleucine but branching is different: in leucine → branching occurs on γ carbon
in isoleucine→ branching occurs on β- carbon
6- Proline: In proline, amino group enters in the ring formation being α-imino group so proline is an α-imino acid rather than α-amino acid
Valine
Leucine
Isoleucine
II- Neutral Sulfur containing amino acids: 2 amino acids
1- Cysteine (Cys): R= CH2SH
What is cystine?
2- Methionine (Met):
III- Neutral, hydroxy amino acids: 2 amino acids 1- Serine (Ser): R= CH2OH
2- Threonine (Thr):
Formation of cystine
The bond S-S in cystine is called: disulfide bond
Serine
Threonine
Cysteine
Methionine
VI- Neutral aromatic amino acids: includes 3 amino acids
1- Phenyl alanine (Phe) :
It’s alanine in which one hydrogen of CH3 is substituted with phenyl group. So it’s called phenyl alanine.
2- Tyrosine (Tyr): - it is P- hydroxy phenyl alanine
- it is classified as phenolic amino acid
3- Tryptophan: contains indol ring
Phenylalanine
Tyrosine
Tryptophan
VI- Neutral amino acids with amide group in R (2):
1- Aspargine (Asn):
2- Glutamine (Gln):
• At physiological PH (7.4), in neutral amino acids, -COOH
group is dissociated forming a negatively charged
carboxylate ion (COO-) and amino group is protonated
forming positively charged ion (NH3+) forming Zwitterion
B- Basic amino acids: contain two or more NH2 groups or nitrogen atoms that act as base i.e. can bind proton. At physiological pH, basic amino acids will be positively charged.
a- Lysine( Lys)
b- Arginine(Arg): contains guanido group:
c- Histidine (His): contains imidazole ring.
guanido
C- Acidic Amino acids:
e.g. a- Aspartic acid (Asp) (aspartate),
b- Glutamic acid (Glu) (glutamate)
at physiological pH, acidic amino acids will carry negative charge .
Summary:
A- Neutral amino acids (15 amino acids)
Classified according to R into
- Aliphatic amino acids including glycine, alanine, valine, leucine,
isoleucine and proline
- Sulfur containing amino acids including: cysteine and methionine
- Hydroxy amino acids including serine and threonine
- Aromatic amino acids including phenylalanine, tyrosine and
tryptophan
- Amino acids containing amide group including aspargine and
glutamine
Lysine
Arginine
Histidine
Basic amino acids (3)
Aspartic acid
Glutamic acid
Acidic amino acids (2)
-At physiologic pH, neutral amino acids are present as Zwitterion
(carry equal positive and negative charges) so it is neutral (not
charged).
- At physiological pH, basic amino acids will be positively charged.
While acidic amino acids are negatively charged
So, Basic and acidic amino acids are called charged amino acids